The present invention relates to chemical mechanical polishing (CMP) and, more particularly, pad assemblies used by CMP machines.
CMP is commonly used in planarizing semiconductor wafers during the fabrication of integrated circuits. A typical CMP system will include an apparatus for holding the wafer, bringing the wafer and a polishing pad into contact, and providing a relative motion between the wafer and polishing pad to polish the wafer surface. In addition, conventional CMP systems provide slurry to aid in the polishing process. In a typical conventional CMP system, the slurry is introduced at the edge of the wafer-polishing pad interface. The slurry typically contains a solution that can react chemically with portions of the wafer surface so that the mechanical action of the polishing pad on the wafer surface can aid the removal of material from the wafer surface.
FIG. 1 shows a top view of conventional polishing pad 10 used in a conventional CMP system. In this system, the slurry is introduced through the pad for better slurry distribution throughout the pad-wafer contact surface. Polishing pad 10 includes a series of grooves 12 arranged to form a rectangular grid. The grooves are typically provided in polishing pad 10 to help channel slurry across the surface of polishing pad 10 during the CMP process. However, the inventors of the present invention have observed that for some applications using polishing pad 10, the removal rate profile can be undesirably non-uniform. As used herein, the removal rate (RR) refers to the rate at which material is removed during the CMP process as a function of the distance along a diameter of the wafer being polished. For example, in a conventional CMP process for polishing a copper layer (i.e., Cu CMP), the inventors of the present invention have observed that the center of the wafer tends to have a lower removal rate. In addition, the inventors of the present invention have observed that conventional Cu CMP processes generally have low stability. That is, the RR performance and the within-wafer non-uniformity (WIWNU) performance degrade as more wafers are polished using a particular polishing pad. As is appreciated by those skilled in the art, CMP process stability is very important in improving yields during the integrated circuit fabrication process. Thus, in a production environment, the polishing pad must be replaced at relatively frequent intervals, thereby undesirably increasing the cost of ownership. Thus, there is a need for a CMP system with improved stability and improved RR and WIWNU performance.
In accordance with the present invention, a polishing pad that improves CMP RR and WIWNU performance is provided. In one aspect of the present invention, a single-layer polishing pad is grooved in a pattern having relatively large radius bends (i.e., greater than the 90xc2x0 bends of conventional rectangular grid grooving). In a further aspect, the groove pattern is designed to match the velocity profile on each point of the pad. This type of groove pattern allows slurry to be more uniformly distributed across the surface of the polishing pad compared to polishing pads having conventional rectangular groove patterns. This improvement in slurry distribution tends to improve RR uniformity and WIWNU. For example, in accordance with this aspect of the invention, the polishing pad can be grooved in a hexagonal pattern, which produces a groove pattern with 120xc2x0 bends. In one embodiment, the grooves do not penetrate all of the way through the polishing pad, thereby maintaining the xe2x80x9cstiffnessxe2x80x9d of the polishing pad, which tends to improve planarization.
Further, the hexagonal grooving pattern, used in conjunction with standard pad conditioning techniques, has yielded startling improvement in stability. The term stability is used in this context to refer to consistent acceptable RR and WIWNU performance over a large number polishing uses. The improved stability reduces cost because significantly fewer polishing pads are needed for polishing a large number workpieces. In addition, the improved stability significantly increases throughput because the polishing pad is not changed as often, thereby decreasing interruptions when polishing a large number of workpieces.
In a further aspect of the present invention, the polishing pad is grooved in a hexagonal pattern by forming a pattern of triangles. For example, six triangles can be arranged to form a hexagon. This aspect of the present invention can be implemented using relatively inexpensive standard polishing pads, which the operator can groove using a standard grooving tool. In particular, the grooving tool is used to form three sets of parallel lines. The second set of parallel lines is formed at an angle of about 60xc2x0 with the first set, and the third set is formed at an angle of about 60xc2x0 with the second set. Thus, a hexagonal groove pattern is relatively easily formed in the polishing pad.